Cellular technologies specified by the 3rd Generation Partnership Program (3GPP) are the most widely deployed in the world. A new step being studied and developed in 3GPP is an evolution of 3G into an evolved radio access technology referred to as Long-Term Evolution (LTE). In LTE, different modes of communication can be used for radio nodes in a cellular network, such as Frequency Division Duplex (FDD), Time Division Duplex (TDD) and half duplex.
In TDD, a single physical channel can be utilized for both uplink and downlink transmissions. The uplink and downlink transmissions are separated in time, in communication between a radio base station and a mobile terminal. I.e. they take place in different, non-overlapping time slots. Therefore, the participating radio nodes are required to switch between transmit mode and receive mode according to a predefined radio frame scheme, thus avoiding that uplink and downlink transmissions occur on that physical channel simultaneously. An example of such a scheme is illustrated in FIG. 1. In this example, a radio frame 100 of 10 ms duration comprises ten sub-frames 0-9 of 1 ms duration each, which can be used for either uplink or downlink transmissions on the same physical channel in a communication.
In FIG. 1, an uplink transmission in dub-frame 5, as indicated by an upwardly directed arrow, is followed by a downlink transmission in sub-frame 6, as indicated by a downwardly directed arrow. A single sub-frame 1 is even divided into a field 102 for a downlink Pilot Time Slot, DwPTS, and a field 104 for an uplink Pilot Time Slot, UpPTS, the fields 102 and 104 being separated by a field 106 denoted Guard Period, GP allowing for the above switch and transition of communication modes. FIG. 1 thus illustrates that both nodes involved in the communication must switch between transmit mode and receive mode in an accurate and synchronized manner to avoid collisions and disturbances on the physical channel used, particularly between uplink and downlink transmissions.
In FIG. 2, a transceiver 200 arranged for communication in TDD mode is shown in a simplified version. The transceiver 200 comprises a transmitter part 210 and a receiver part 220. The transmitter part 210 is of a direct-conversion type, which is widely used for radio base stations (RBSs) in LTE. The transmitter part 210 comprises a transmitter local oscillator (TX LO) 212 connected to a mixer 214. The receiver part 220 comprises a receiver local oscillator (RX LO) 222 connected to a mixer 224. The transceiver further comprises a mode switch 230 and an antenna 240. The mode switch 230 is used to either connect the transmitter part 210 or the receiver part 220 to the antenna 240. Thereby, the same physical link may be used for either uplink or downlink communication. By uplink communication is meant transmission from a mobile terminal to an RBS. At uplink communication the mode switch 230 is set such that the receiver part 220 of the transceiver 200 of the RBS is connected to the antenna. By downlink communication is meant transmission from an RBS to a mobile terminal. At downlink communication the mode switch 230 is set such that the transmitter part 210 of the transceiver 200 is connected to the antenna. The above described transmitter part 210 is called a direct-conversion transmitter since no intermediate frequency (IF) stage is used, i.e. no up-conversion stage and IF filters have to be used. Instead, the baseband signal is directly modulated onto the radio frequency (RF) carrier.
Accordingly, an input signal to be transmitted is fed to the input of the transmitter 210 and mixed in the mixer 214 with a local oscillator signal fed from the TX LO 212. Further, if the transceiver is in transmission mode, i.e. if the mode switch 230 has connected the transmitter part 210 to the antenna 240, the modulated signal can be transmitted on the downlink to mobile terminals via the antenna 240. When the transceiver is in reception mode, the mode switch has connected the receiver part 220 to the antenna 240, and an uplink signal can be received at the receiver part 220. Further, the received uplink (UL) radio signal can be down-converted in the mixer 224 by being mixed with a corresponding RX LO signal. Thereby, the carrier wave is subtracted and the base band signal remains. The RX LO signal may be generated by a separate RX LO or by the TX LO.
Such transceivers may experience disturbances, especially for signals received on the uplink. Consequently, there is a need to decrease disturbances at uplink communication for transceivers operating in TDD mode.